The present invention relates to a modulation/demodulation system, and more particularly to a complex signal modulation/demodulation method and system for realizing a quadrature modulation/demodulation technique used in a digital mobile radio system by a complex signal processing.
In recent years, a digital radio system has been investigated in order to enhance the reliability of mobile communication. In the digital radio system, a carrier signal is modulated by information data to be transmitted and the modulated signal is transmitted and received. Though there are various modulation systems, a quadrature modulation system is widely used. In the quadrature modulation system, the amplitude and phase of a carrier signal are both modulated. For this purpose, two components of a carrier signal, that is, the in-phase component (I) and the quadrature phase component (Q) thereof are modulated. Usually, two modulation signals used for modulating I and Q can be obtained by time-dividing a signal of one system.
FIG. 1 shows an example of the conventional quadrature modulation system constructed with analog circuits. In the figure, reference numeral 1 denotes a constellation mapping circuit, numerals 2 and 3 low pass filters (LPF's), numerals 4 and 5 frequency mixers, numeral 6 a 90.degree. phase shifter, numeral 7 a carrier signal generator, and numeral 8 a signal adder. Data signals I and Q to be transmitted are inputted to the constellation mapping circuit 1 which in turn converts signals with logical levels of 0 and 1 into signals with amplitude levels of 1 and -1. The converted signals are subjected to a waveform shaping and band limitation process by the low pass filters 2 and 3, respectively. In the frequency mixers 5 and 4, the processed signals are respectively mixed with a quadrature phase carrier signal which is generated from the carrier signal generator 7 and an in-phase carrier signal which is obtained by inputting the quadrature phase carrier signal to the 90.degree. phase shifter 6. Then, the mixed signals are added together by the signal adder 8, thereby performing a quadrature modulation. A quadrature demodulation can be realized by performing an operation reverse to that in the quadrature modulation shown in FIG. 1.
In recent years, an attempt to construct a quadrature modulator/demodulator with digital circuits has been made owing to the advance of digital circuit technology. A construction in the case of a digital processing is substantially the same as that in FIG. 1 but has additionally a digital-to-analog (DA) converter and an analog filter (for elimination of the aliasing component of a digital signal) at the final output stage.
The prior art relevant to the quadrature modulator/demodulator includes JP-A-3-106188.